* Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > On Sun, 2010-03-28 at 23:37 -0400, Matt Turner wrote: > > include/asm-generic/bitops/sched.h says > > /* > > * Every architecture must define this function. It's the fastest > > * way of searching a 100-bit bitmap. It's guaranteed that at least > > * one of the 100 bits is cleared. > > */ > > > > arch/alpha/include/asm/bitops.h says > > /* > > * Every architecture must define this function. It's the fastest > > * way of searching a 140-bit bitmap where the first 100 bits are > > * unlikely to be set. It's guaranteed that at least one of the 140 > > * bits is set. > > */ > > > > Is the guarantee that one of the first 100-bits set (and that the last > > 40 are useless?), or 140-bits? If it's just the first 100 bits we care > > about, then the alpha version needs to be fixed. > > > > I'm just checking this out, because gcc produces horrendous code for > > sched_find_first_bit on alpha. I rewrote it in assembly and it's > > better than 4 times faster. > > > > Also, is it even worth optimizing that function? It looks like it's > > only used in kernel/sched_rt.c. > > (might help if you CC the scheduler people on scheduler functions :-) > > Right, so it used to be 140 bits with the old O(1) scheduler, currently > (as you noted) sched_rt is the only user left and will therefore only > care about the first 100 bits. > > As it stands I think it might still make sense to optimize this as for > rt loads it still on a hot path. > > As to the 100 vs 140 length, would it really make much of difference to > shorten the implementation to 100? If not I'd leave it at 140. > > Ingo, any comments? I guess getting below the 128 bits boundary would shave an instruction and a branch off or so? Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-alpha" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
